This invention is related to the power generation industry and, more particularly to the field of monitoring power generators.
Electrical power can be generated by electromagnetic induction using a generator having a rotor which in response to mechanical energy supplied to the generator turns within a stator core. The stator core typically is formed from laminations secured together by a plurality of through-bolts. As the rotor turns within the stator core, a significant amount of flux develops which passes within the core through the through-bolts. As a result of the flux so generated, a voltage is induced across each of the through-bolts within the stator core. If a closed conductive path develops within the core, the voltage across any one of the through-bolts can induce current that will flow through the path within the core. The current can result in serious damage to the core laminations. Thus, to avoid damage to the laminations of the core, the through-bolts securing the laminations to one another often are insulated.
Due to stress from sustained operation of the generator and wear over time, the insulation surrounding any of the through-bolts in the core can breakdown so that the insulation loses its insulating properties thereby causing an unintended ground. The breakdown in insulation can result from a clean tear that extends through the insulation, for example, or from a fraying of portions of the insulation. Along the insulation surface exposed by the breakdown, a conductive path can arise as, for example, as a result of conductive particulate matter on the exposed surface. There can also be arching. In any event, one or more grounds result from the loss of the insulation""s insulating properties and a low-impedance path is thereby created for current to flow in the through-bolt and the core laminations. If more than one ground develops owing to the breakdown in the insulation of more than one through-bolt or multiple breakdowns within the insulation of even one through-bolt, multiple grounds will result. When more than one ground occurs, a closed conductive path is formed that can carry core-damaging current induced by the voltage or potential difference produced in each of the through-bolts by the significant amount of flux passing through the core as a result of electromagnetic induction. The current flow, if it occurs, then causes serious damage to the laminations of the stator core that the insulation surrounding the through-bolts was intended to prevent.
In view of the foregoing, the present invention advantageously provides an apparatus to protect against core-damaging breakdowns in the insulation surrounding portions of a plurality of through-bolts used to secure together the laminations forming a generator core. A distinct advantage of the apparatus is the ability to detect even a single breakdown. This allows the breakdown to be detected prior to any damage to the core. Specifically, each breakdown in insulation causes a ground. If two or more grounds develop, a low-impedance path develops in the core along which core-damaging current can pass. Therefore, by detecting the occurrence of even a single ground, corrective action can be taken before a second ground occurs. Thus, the apparatus indicates the potential for core damage in time for corrective measures before any core damage has occurred. By detecting the first occurrence of insulation breakdown, the substantial cost that would be incurred in replacing an entire current-damaged core is avoided.
Yet a further advantage of the present invention is the ability to identify which through-bolt among a plurality of through-bolts has experienced an insulation breakdown. This allows for corrective action such as replacing the insulation of the affected through-bolt to be directed to the particular insulation of a through-bolt causing the problem. Thus, in addition to providing for the avoidance of the substantial cost of replacing an entire current-damaged core, the apparatus also provides an indication of where corrective action should be directed before any core damage has occurred.
According to the present invention, the apparatus preferably includes a signal source positioned adjacent to or remotely from the generator core. The signal source preferably is connected to an exposed, uninsulated portion of each through-bolt. The signal source is positioned to generate a preselected signal in response to an absence of any breakdown in insulation of any of the through-bolts. The signal source, accordingly, generates an alternative signal in response to a breakdown in the insulation of one of the through-bolts.
The apparatus, moreover, preferably includes at least one signal regulator to regulate the level of the signals generated by the signal source and thereby maintain the signals within predetermined ranges. In addition, a signal sensor is responsive to the signal source to sense a signal generated in response to a breakdown in the through-bolt insulation of at least one through-bolt.
These elements advantageously cooperate to detect a breakdown in the insulation surrounding any one of the through-bolts by detecting even a single ground that develops in the generator core. As already noted, when a breakdown in the insulation of any of the through-bolts occurs, a ground develops. Because of the significant amount of flux passing through the core as a result of electromagnetic induction during operation of the generator, there is voltage or potential difference across each of the through-bolts. Therefore, if two or more grounds develop as a result of a breakdown in the insulation of more than one through-bolt (or multiple breakdowns in even one through-bolt), then a closed, conductive path arises within the core through which current will flow in response to the voltage or potential difference in the through-bolt. The current flow, if it occurs, can cause serious damage to the laminations of the generator core as also already noted.
To avoid damage to the generator core laminations, the apparatus of the present invention detects even a single ground that develops as a result of breakdown in the insulation of a through-bolt. Specifically, the apparatus provides a ground detector circuit in which a signal source is electrically connected to each of the through-bolts. A signal sensor is electrically connected to the signal source. If a single ground occurs due to a breakdown in the insulation of one of the through-bolts, the circuit closes (i.e., the signal source and/or signal sensor are connected to ground) and a signal indicating a breakdown in isolation is generated and detected. Preferably, the signal source is a voltage source and the signal sensor is a current sensor. If a ground develops as a result of insulation breakdown, current is generated through the completed circuit and sensed as a warning that a breakdown in through-bolt insulation has occurred. Conversely, if the insulation surrounding each of the through-bolts is in good working condition, then the circuit is open and no current is induced by the voltage.
More generally, the signal source generates a first signal in response to no breakdown in insulation of any of the through-bolts and, additionally, generates a second signal in response to a even a single breakdown in insulation of any of the through-bolts. To regulate the precise levels of the first and second signals so generated, at least one signal regulator is also provided. Preferably, the at least one signal regulator is provided by a plurality of resistors. Each of the plurality of resistors, moreover, is preferably connected in series between the signal source and a corresponding through-bolt. The resistors substantially prevent current flow between through-bolts via the ground detector circuitry. The resistors, moreover, prevent a solid ground of the through-bolt if the insulation of the ground detector circuit should fail or if the leads from the through-bolts to the ground detector circuit should fail.
In the context of a signal source provided by a direct current voltage that generates a signal in the form of a current in response to a breakdown in insulation of one of the through-bolts, the signal in the form of a direct current, Idc, is given by the following equation: Idc=Vdc/(R+Rgnd), where R is the ohmic resistance of a resistor connected in series between the signal source (i.e., direct current voltage) and Rgnd is the effective ohmic resistance to ground of the through-bolt experiencing a breakdown in insulation. The specific level of the current Idc can be varied according not only to whether a breakdown in the insulation of a through-bolt has occurred but also in accordance with which of one of the through-bolts has experienced a breakdown in insulation. Thus, more generally, according to the present invention, the apparatus detects a breakdown in insulation in one through-bolt from among a plurality of through-bolts positioned to secure a plurality of core laminations together and provides means for identifying which among the plurality of through-bolts has experienced a breakdown in through-bolt insulation. More generally, the signal source thus generates a first preselected signal in response to no breakdown in insulation among the plurality of through-bolts and a second preselected and regulated signal generated by the signal source and regulated by the signal regulator. The second preselected and regulated signal preferably varies according to which through-bolt experiences a breakdown in insulation. The apparatus preferably also includes an indicator circuit responsive to the signal sensor to generate an indication or otherwise generate an alarm when a signal is generated in response to a breakdown in the insulation surrounding at least one of the through-bolts in the generator core.
Various method aspects for detecting a breakdown in through-bolt insulation surrounding a through-bolt positioned within a generator core are also provided by the present invention. The method preferably includes supplying a current to the generator core, passing the current through the generator core when a breakdown in through-bolt insulation occurs, and detecting the current which is passed through the generator in response to the breakdown in through-bolt insulation. The method aspects, moreover, can further include passing a current through the generator core such that the level of the current passing through the generator core is responsive to whichever of the through-bolts has experienced a breakdown in through-bolt insulation.
In addition, the method aspects of the present invention provide for detecting insulation breakdown in an electrical system by positioning at least one insulated conductor, defining a first conductor positioned within a second conductor, and providing a signal responsive to a breakdown in the insulation of the first conductor. The method also includes providing a plurality of insulated conductors in distinct regions of the second conductor and detecting the region in the second conductor in which a breakdown in insulation occurs.